The argument that HIV does not lead to Aids - put forward by South African President Thabo Mbeki among others - has no basis in fact, say scientists.

Tests on monkeys have come "as near as we're ever going to get" to proving that HIV does cause Aids, said Ruth Ruprecht of Harvard Medical School.

If you look at HIV positive people and HIV negative people, the positive ones have a hugely increased risk of Aids-related illnesses

Julian Meldrum, National Aids Trust

The results of the study come as another group of scientists claims that a lack of understanding about how vaccines work is hampering efforts to develop ways of preventing Aids.

More than 11m people have now died of Aids in Africa, but President Mbeki is a leading proponent of the view - rejected by most of the scientific community - that HIV does not cause the disease.

Professor Peter Duesberg, of the University of California at Berkeley, has argued that it is impossible to confirm HIV leads to Aids, and says drugs may be the original cause of the disease.

But Ruprecht set out to prove the link by isolating the pure DNA form of the SIV virus, HIV's close relative that causes Aids in monkeys.

This DNA was then injected into six infant and six adult macaques. All 12 animals made antibodies to the virus, but three of the infants and one of the adults died from Aids, reports New Scientist magazine.

Ruprecht said that short of deliberately injecting purified HIV into humans, this was "as near as you are ever going to get to proving HIV causes Aids".

Further confirm

Julian Meldrum of the National Aids Trust in the UK said the Ruprecht study seemed to further confirm that HIV was the cause of Aids.

"If you look at HIV positive people and HIV negative people, the positive ones have a hugely increased risk of Aids-related illnesses," he said.

Drugs can alleviate the symptoms of Aids - but there is no vaccine and no cure

However, Dr Charles Thomas, of the Helicon Foundation in San Diego, California, is among scientists who have called for more evidence that HIV causes Aids.

He questioned the results of the Ruprecht study, saying: "SIV is something quite different from HIV - that has never been isolated."

And Harry Rubin, professor of cell biology at the University of California at Berkeley, who has also signed a demand for more research, said: "I don't see that this is conclusive evidence.

"I now think there is pretty good evidence that HIV has something to do with Aids, but I think it is a multi-factorial problem.

"The closed-mindedness of both ends of the spectrum is really not convincing."

The battle to produce an Aids vaccine continues, whatever the agent that causes the disease.

But Philippe Kourilsky, director of the Pasteur Institute in Paris, said not enough work had been done on other vaccines, such as measles and polio, to establish why they work.

The belief that successful vaccines work by producing antibodies is almost certainly wrong, added Neal Nathanson, director of the US Office of Aids Research.